Across a field study and four experiments, we examine how social norms and scrutiny affect decisions about adding members of underrepresented populations (e.g., women, racial minorities) to groups. When groups are scrutinized, we theorize that decision makers strive to match the diversity observed in peer groups due to impression management concerns, thereby conforming to the descriptive social norm. We examine this first in the context of U.S. corporate boards, where firms face pressure to increase gender diversity. Analyses of S&P 1500 boards reveal that significantly more boards include exactly two women (the descriptive social norm) than would be expected by chance. This overrepresentation of two-women boards — a phenomenon we call "twokenism" — is more pronounced among more visible companies, consistent with our theorizing around impression management and scrutiny. Experimental data corroborate these findings and provide support for our theoretical mechanism: decision makers are discontinuously less likely to add a woman to a board once it includes two women (the social norm), and decision makers' likelihood of adding a woman or minority to a group is influenced by the descriptive social norms and scrutiny faced. Together, these findings provide a new perspective on the persistent underrepresentation of women and minorities in organizations.